Capacitive pressure transducer structure and method for manufacturing the same

Abstract: The invention concerns a capacitive pressure transducer structure and a method for manufacturing the same. The transducer structure comprises a contiguous diaphragm structure (2), which at least in some parts is conducting to the end of forming a first electrode (4) of a transducing capacitor, a substrate (3) which is permanently bonded to a first surface of said diaphragm structure (2) and comprises a second electrode (5) of said transducing capacitor, spaced at a distance from and aligned essentially at said first electrode (4), and a silicon structure (1), which is permanently bonded to a second surface of said diaphragm structure (2), incorporating a space (21) suited to accommodate the deflection of said first electrode (4). According to the invention, the angle α formed between vertical walls (11) of said space (21) and said first electrode (4) is smaller than or equal to 90°, and the material surrounding said space (21) is silicon or doped silicon.

Description: The present invention relates to a capacitive pressure transducer structure according to the preamble of claim 1. The invention also concerns a method for manufacturing said capacitive pressure transducer structure. With regard to the prior art, reference is made to the following publications: US patents [1] U.S. Pat. No. 4,386,453 (Gianchino et al.) [2] U.S. Pat. No. 4,257,274 (Shimada et al.) [3] U.S. Pat. No. 4,332,000 (Petersen) [4] U.S. Pat. No. 4,390,925 (Freud) [5] U.S. Pat. No. 3,397,278 (Pomeranz) [6] U.S. Pat. No. 4,589,054 (Kuisma) [7] U.S. Pat. No. 4,628,403 (Kuisma) [8] U.S. Pat. No. 4,594,639 (Kuisma) [9] U.S. Pat. No. 3,288,656 (Nakamura) Cited publications [1]-[5] are related to a capacitive pressure transducer structure in which a silicon diaphragm acting as a moving first electrode flexes toward a second, fixed metal electrode so as to permit the electrodes approach to each other with increasing pressure. The distance between the electrodes is a function of the imposed pressure. Cited publications [6], [7] and [8] disclose a capacitive pressure transducer structure in which the silicon diaphragm acting as the first electrode flexes away from the fixed metal electrode with increasing pressure. Cited publication [9] describes first time in the art a method for interbonding two silicon wafers using a technique called fusion bonding. Prior-art capacitive pressure transducer structures based on arranging the electrodes to be closest to each other at the lowest pressure are disclosed in cited publications [6], [7] and [8]. Such a pressure transducer structure has a very wide operating pressure range. The capacitance rate-of-change with respect to pressure in the transducer structure is highest at low pressures when the electrodes are very close to each other, thus making the pressure measurement extremely accurate also at low pressures. Owing to the use of a vacuum chamber in the structure, no via holes for the conductors are required which helps attaining a stable vacuum. The vacuum chamber is sealed by a glass plate or silicon wafer coated with a glass layer. The first electrode of the transducer capacitor is thus formed by the smaller-area end of the conical vacuum chamber. Consequently, the conical vacuum chamber widens outward from the transducer diaphragm. Disadvantages of the prior-art technology structures are: 1. Owing to the different coefficients of thermal expansion in silicon and glass, the flexible diaphragm is subjected to stresses which result in a substantial temperature dependence of the transducer. 2. Residual gases in the vacuum chamber cause long-term instability. 3. Transducer size has been excessively large for a low-cost volume product. The number of transducers per processed wafer has been too small. 4. The transducer structures have been excessively complicated requiring the use of several lithography processes. To improve the yield in the manufacturing process of the transducer, its structure and manufacturing process should be the simplest possible. 5. As the diaphragm acting as the active transducer member is the smaller end of the conical vacuum chamber, the other end, the larger-area end has been the crucial factor increasing the area of the transducer element. In other words, when a certain design area has been selected for the transducer diaphragm, due to the limitations of the transducer technology employed, the size of the larger-area end of the vacuum chamber has caused a drastic reduction in the number of transducers fitting on a wafer. It is an object of the present invention to overcome the disadvantages of the above-described prior-art technology and to achieve an entirely novel type of capacitive pressure transducer structure and a method for manufacturing the same. The invention is based on: 1) sealing the vacuum chamber of the transducer structure by fusion bonding two silicon wafers together, and 2) thinning the silicon wafer intended to form the flexible silicon diaphragm subsequent to the sealing of the vacuum chamber. More specifically, the capacitive pressure transducer according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the method according to the invention is characterized by what is stated in the characterizing part of claim 5. The invention offers significant benefits. The manufacturing process and structure of the transducer are extremely uncomplicated. The transducer structure has a very small size and low manufacturing cost. The transducer provides a wide operating range. Minimization of the transducer size achieves the lowest possible cost of the transducer structure. Owing to the simple manufacturing process, a high yield and minimized manufacturing costs are attained. The benefits of the invention include the possibility of maximizing the vacuum chamber volume, while the transducer area is simultaneously retained the smallest possible. As known, residual gases in the vacuum chamber cause temperature dependence of the transducer zero point, which may be decreased by maximizing the vacuum chamber volume. This invention discloses the first time in the art a transducer structure with a vacuum chamber construction made from no other materials except silicon. As known, structures having the vacuum chamber sealed by glass are hampered by the fact that glass easily absorbs gases which may then be released into the vacuum chamber thus causing long-term instability of the transducer. The footprint of the present transducer structure is minimized, because the manufacturing process of the vacuum chamber wastes no wafer area in the slanted chamber walls as is the case with the prior-art structures [6],[7]. A further benefit of the present invention is that a hermetic bond is attained in the silicon-silicon interface, whereby hermeticity can easily be assured using prior-art surface treatments in a conventional manner [9]. The anodic bonding of the glass-silicon interface in the present structure is not subject to the hermeticity requirement. Some of these advantages may be seen in the sensors disclosed in the IRE WESCON CONVENTION RECORD, vol. 35, November 1991, NORTH HOLLYWOOD US, pages 419-426 J. BRYZEK 'Silicon Pressure and Acceleration Sensors'. The invention is next examined in greater detail with the help of exemplifying embodiments illustrated in the appended drawings, in which Figure 1 is a side view of an embodiment of the capacitive pressure transducer according to the invention; Figure 2 is a side view of a second embodiment of the capacitive pressure transducer according to the invention; Figure 3 is a side view of a third embodiment of the capacitive pressure transducer according to the invention; Figure 4 is a side view of a fourth embodiment of the capacitive pressure transducer according to the invention; Figure 5 is a top view of the silicon-glass interface in the structure illustrated in the diagram of Fig. 4; Figures 6a - 6g are side views of the different steps of the manufacturing method according to the invention; Figure 7 is a side view of a capacitive pressure transducer according to the invention provided with overpressure protection; and Figure 8 is a side view of a capacitive differential pressure transducer according to the invention. With reference to Figs. 1, 2, 3 and 4, exemplifying embodiments of pressure transducer structures according to the present invention are illustrated having a design in which the silicon diaphragm is flexed away from the fixed metal electrode with increasing pressure and having the vacuum chamber sealed with a silicon wafer. The manufactured transducer structures are illustrated as separate transducers. In the actual manufacturing process, however, the transducers are made as an array on circular wafers from which the individual transducers are separated by sawing. In a preferred embodiment of the invention, the vacuum chamber is made into silicon using conventional micromechanical techniques starting from a thicker silicon wafer and then sealing the chamber by a thinner silicon wafer which is subjected to further thinning after the silicon wafers are bonded. Such a manufacturing method according to the invention is illustrated in Figs. 6a - 6g. With reference to Fig. 1, the transducer embodiment shown herein is made by first using conventional micromechanical techniques to work into a thick silicon wafer 1 a vacuum chamber 21 having slanted walls 11 which, however, do not consume extra silicon footprint which would be detrimental to the goal of attaining a small physical size of the transducer. In accordance with the invention, the angle α formed between the wall 11 and a diaphragm 4 formed into the thin silicon wafer 2 so as to act as the moving electrode of the transducing capacitor is smaller than or equal to 90°. The angle α is defined as the angle formed when the diaphragm 4 is not subjected to external pressure. The width of the vacuum chamber 21 determines the width of the flexible diaphragm 4 which is one of the transducer's key dimensions in a given application. To the thicker wafer 2 into which the vacuum chamber 21 is formed is hermetically attached a thin silicon wafer 2 by fusion bonding. Prior to bonding, the flat silicon surfaces are treated by conventional silicon surface wash processes, after which the wafers are superimposed in a vacuum and the bond is sealed by a heat treatment. In Fig. 1 the thinned silicon wafer 2 forms both the flexible diaphragm 4 and the air gap 23 which determines the zero-point capacitance. The depth of the vacuum chamber 21 is limited by the thickness of the thicker silicon wafer which typically ranges from 500 µm to 1500 µm. The diaphragm 4 or at least its lower surface is doped conducting. To this end, the typical bulk doping level is 10 18 impurity atoms/cm 3 . The thin flexed diaphragm 4 remains adhered to the thicker silicon wafer 1 during the entire thinning process of the diaphragm area, which is essential for attaining a high yield through keeping the diaphragm intact during the thinning process. While the thickness of the flexible diaphragm 4 must be known to determine the pressure sensitivity of the transducer, the measurement of the diaphragm thickness is impossible by conventional methods as the diaphragm 4 is an integral part of the thicker wafer. However, the deflection profile of the diaphragm 4 can be measured optically and then the diaphragm thickness can be computed from the measured deflection. The thinned, flat silicon surface is next furnished with a metal electrode 5 and a bonding pad 3 thereof by, e.g., anodic bonding, whereby a glass layer deposited on the silicon wafer provides isolation. With reference to Fig. 2, a transducer structure similar to that described above is illustrated with, however, the air gap 23 located differently from that shown in Fig. 1. The air gap 23 is here made in the glass layer 3, while in the structure shown in Fig. 1 it was made in the silicon of the wafer 2. The most critical dimension of the transducer is the depth of the air gap, and consequently, its processing in a homogenous glass layer is easier to control than for silicon in which the etching rate of silicon is influenced by, i.a., the resistivity of the silicon wafer. With reference to Fig. 3, a transducer structure is shown in which the substrate for the metal electrode 5 is formed by a silicon wafer 3 having a thin glass layer 6 deposited on it in the manner described in cited reference [6]. The air gap 23 is formed by etching in the silicon of the wafer 2. With reference to Fig. 4, a transducer structure is shown in which the air gap and the dielectric isolation of the wafer 2 from the metal electrode 5 is provided by the thin glass layer 6. The section plane of the diagram is coincident with the communication channel of the external pressure. With reference to Fig. 5, a transducer structure is shown in a top view illustrating the interface between the silicon wafer providing the substrate for the metal electrode 5 and the thinned silicon wafer. The silicon-glass interface is denoted by reference numeral 15. The communication channel 13 of the external pressure is formed at the metallization 5 of the other capacitor electrode. With reference to Figs. 6a - 6g, the transducer is manufactured comprising: a) Processing a typically 500-1500 µm thick silicon wafer 1 by conventional lithography methods to comprise a vacuum chamber 21 of thickness 100-1000 µm. Silicon is etched by, e.g., 3-50 % potassium hydroxide solution giving an etching rate of 0.5-1 µm/min typical. In a wafer of normal silicon, the walls of the chamber 23 are etched slanted so as to shape the chamber 23 into a truncated cone. b-c) Attaching to the silicon wafer 1 with the vacuum chambers by fusion bonding another silicon wafer 2 of 200-400 µm thickness typical. Prior to the fusion bonding step, the silicon surfaces are treated by, e.g., hot nitric acid, RCA wash or H 2 SO 4 /H 2 O 2 wash. d-e) Thinning the silicon wafer 2 to an extremely thin thickness of 1-100 µm, typically 20 µm. The thinned wafer 2 remains during the entire thinning process attached to the thicker wafer. When desired, the wafer 2 can further be thinned at the vacuum chamber 21 to the end of forming an air gap under the transducing diaphragm 4 relative to the other electrode of the transducing capacitor. f-g) Bonding both interbonded silicon wafers with the ready-processed flexible diaphragm 4 to a substrate 3 incorporating an isolated metal electrode 5 of the transducing capacitor. With reference to Fig. 7, a transducer structure similar to that shown in Fig. 1 is illustrated with an overpressure protection 17. Such an overpressure protection 17 is formed by etching a part of the vacuum chamber bottom to a depth corresponding to the desired maximum permissible deflection of the diaphragm 4. Thus, the transducer can be made to stand also high overpressures during which the diaphragm is deflected against the overpressure protection 17 without rupturing. With reference to Fig. 8, a differential pressure transducer structure is shown having the chamber provided with a drilled or etched hole 19 to the end of permitting the chamber 23 to communicate with the external pressure. Contacts to the transducing capacitor electrodes can also be furnished by virtue of contacts made through the glass substrate layer 3. Via holes for the contacts can be implemented either by mechanical or chemical processing steps. The via holes are made conducting by sputter deposition of a suitable metal in them to the end of forming the electrical contacts. Such an arrangement can be used to pack more transducers on a wafer.

Claims: A capacitive pressure transducer structure comprising a contiguous diaphragm structure (2), which at least in some part is conducting so as to form a first electrode (4) of a transducing capacitor, a substrate (3), which is permanently bonded to a first surface of said diaphragm structure (2), comprising a second electrode (5) of said transducing capacitor, spaced at a distance from and aligned substantially at said first electrode (4), and a silicon structure (1), which is permanently bonded to a second surface of said diaphragm structure (2), and which defines a space (21) suited to accommodate a deflection of said first electrode (4), characterised in that the angle α formed between side walls (11) defining said space (21) and said first electrode (4) is smaller than or equal to 90°, and the material surrounding said space (21) is silicon or doped silicon. A capacitive pressure transducer structure as defined in claim 1, characterised in that a projection (17) arranged to limit the deflection of said first electrode (4) is provided in the interior of said space (21). A capacitive pressure transducer structure as defined in claim 1 or claim 2, characterised in that said space (21) is a vacuum chamber. A capacitive pressure transducer structure as defined in claim 1, 2 or 3, characterised in that said space (21) is provided with a channel (19) for communication with an external pressure. A method for manufacturing a capacitive pressure transducer structure, in which method is formed a transducer structure comprising a contiguous diaphragm structure (2), which at least in some part is conducting so as to form a first electrode (4) of a transducing capacitor, a substrate (3), which is permanently bonded to a first surface of said diaphragm structure (2), comprising a second electrode (5) of said transducing capacitor, spaced at a distance from and aligned substantially at said first electrode (4), and a silicon structure (1), which is permanently bonded to a second surface of said diaphragm structure (2), and which defines a space (21) suited to accommodate a deflection of said first electrode (4), characterised in that the method comprises attaching said diaphragm structure (2) over said space (21) formed in said silicon structure (1) in substantially vacuum conditions using fusion bonding techniques, thinning said diaphragm structure (2) by micromechanical methods so as to form said first electrode (4), and attaching said silicon structure (1) now incorporating said first electrode (4) to said substrate (3) incorporating said second electrode (5). A method as defined in claim 5, characterised in that in the interior of said space (21) is formed a projection (17) adapted to limit the deflection of said first electrode (4). A method as defined in claim 5 or claim 6, characterised in that said space (21) is adapted to be a vacuum chamber. A method as defined in claim 5, 6 or 7, characterised in that said space (21) is adapted to communicate via a channel (19) with an external pressure.

Inventors: KANKKUNEN TARJA, KANKKUNEN, TARJA
Applicants: VAISALA OY
Application year: 1994
Main classification: G01L9
IPC codes: G01L9, H01L29